Modulation amplifier for radio transmitters

ABSTRACT

A modulation amplifier includes a signal converter, a summator and a plurality of high frequency stages (3 1-m  or 4 1-n ). The high frequency stages (3 1-m ) are driven by equiphased control signals (s 1-m ); the high frequency stages (4 1-n ) are driven by control signals (t 1-n ) of different phase Φ 1-n . The high frequency stages (3 1-m  or 4 1-n ) can be selected in an arbitrary manner by the control signals (s 1-m  or t 1-n ). The summator adds the high frequency output signals (x 1-m  or y 1-n ) of the high frequency stages (3 1-m  or 4 1-n ). The output signal can be conveyed, for example, to a low pass and a subsequent load. The phases (Φ 1-n ) of the output signals (y 1-n ) of the high frequency stages (4 1-n ) correspond to those of the control signals (t 1-m ). So many high frequency stages (3 1-m ) are selected that there is at the output of the summator a high frequency oscillation, whose amplitude changes by steps in accordance with the low frequency signal. In addition, so many high frequency stages (4 1-n ) with different phases (Φ 1-n ) are selected that the sum of the outputs (y 1-n ) corresponds to the difference of the amplified amplitude of the low frequency signal and the coarse approximation of the low frequency signal to the sum of the output signals (x 1-m ).

TECHNICAL FIELD

The invention relates to the field of radio transmitter technology. Itis based on a modulation amplifier according to the preamble of thefirst claim.

STATE OF THE ART

Such a modulation amplifier is already known from the European patentEP-B1-0 083 727.

A modulation amplifier of this kind comprises a signal converter, whichconverts a low frequency input signal into a multiple of controlsignals, a plurality of high frequency stages, which are driven by thecontrol signals, and a high frequency oscillator.

The high frequency stages can be turned on and off independently andsend a high frequency signal to their outputs. The output signals ofthese high frequency stages are added in a summator. So many stages arealways driven or turned on now that the amplitude of the high frequencycomposite signal changes by steps in accordance with the low frequencyinput signal. In this manner the low frequency signal modulates theamplitude of a high frequency oscillation. One can simultaneouslyincrease the power by adding the output signals of the high frequencystages, which can amount, e.g., to a few hundred volts.

There are also means that serve to approximate the difference of thestep-shaped composite signal and the low frequency amplitude to beamplified.

These means comprise in the aforementioned document four high frequencystages with binarily weighted amplitudes of 1/2, 1/4, 1/8 and 1/16 ofthe amplitude of the other 31 coarse stages.

The drawback with this arrangement is that different types of stageshave to be used and a lot more stress is put over time on the stageswith the binarily weighted amplitude than on the other similar coarsestages. Since the stages are not stressed uniformly, the modulationamplifier is inherently susceptible to interference. If binarilyweighted stages are defective, the other stages cannot assume theirtask. Consequently there is a range of high frequency noise that has adetrimental effect on the neighboring channels and decreases the lowfrequency quality.

There exists a special kind of amplitude modulation--the socalledampliphase modulation--from various older publications (H. Chireix:"High Power Outphasing Modulation" in Proceedings of the Institute ofRadio Engineers, vol. 23, no. 11, November 1935; G. Clark: "A Comparisonof Current Broadcast Amplitude Modulation Techniques", in IEEETransactions on Broadcasting, vol. BC-21, no. 2, June 1975; D. R.Mousson: "Ampliphase for Economical Super-Power AM Transmitters", inBroadcast Transmitter Engineering; F. J. Casadevall: "The LINCTransmitter", in RF Design, February 1990).

This kind of modulation is used for a transmitter in the European patentapplication EP-A2-0 273 827. This transmitter comprises two partialtransmitters, which are driven with different phases, a unit, whichcontrols the phases of the transmitters as a function of an inputsignal, and a summation network, which adds the output signals of bothpartial transmitters, in such a manner that an amplitude and phasemodulated signal, whose amplitude changes in accordance with the inputsignal, is sent to the subsequent antenna. By dividing the transmitterinto two partial transmitters the total transmitting power is dividedbetween the two partial transmitters, so that each partial transmitterhas to exhibit only a part of the total power.

PRESENTATION OF THE INVENTION

The object of the present invention is to provide a modulationamplifier, whose means for fine approximation do not represent inherentsources of error.

This problem with a modulation amplifier of the aforementioned kind issolved by means of the features of the first claim.

Thus, the essence of the invention is that the means for fineapproximation and coarse approximation comprise similar stages, wherethe fine stages are driven with different phases. Thus, the coarse andfine stages differ only in the phases of the selection and outputsignals. Their internal construction, however, is identical. The sameapplies to the output amplitudes and output frequency.

In a first embodiment the number of stages that are driven with the samephase is larger than the number of stages that are driven with differentphases. The result is a modulation amplifier, where the summed outputsignal of the equiphase stages exhibits an essentially step-shapedamplitude response, whereas the fine approximation is done by stageswith different phases.

In a second embodiment the number of stages that are driven with thesame phase is smaller than the number of stages that are driven withdifferent phases. In the extreme case even all stages with differentphases are selected. Thus, the result is a modulation amplifier, whoseoutput signal amplitude varies in accordance with a low frequency inputsignal and is approximated by means of the sum of a plurality ofequifrequent oscillations with different phase.

Other embodiments follow from the dependent claims.

The advantage of the construction according to the invention is that allof the used stages exhibit the same construction and the same outputamplitudes and are driven only by different control signals. The resultis a modulation amplifier, which is characterized by simplicity andsturdiness. The similarity of the stages also allows greater flexibilitywith respect to the construction of the amplifier, so that not onlyamplifiers, in which only the fine approximation is realized by means ofout-of-phase signals, but also amplifiers, in which the totalapproximation is realized by means of out-of-phase signals, can beconstructed with the same means.

With the construction of the invention the error source, which afflictsa modulation amplifier due to non-uniform stress on all stages, isavoided.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following the invention is explained in detail with reference tothe embodiments and the drawings.

FIG. 1 is a vector drawing of the addition of two signals with differentphase.

FIG. 2 is a block diagram of a modulation amplifier, which exhibits theinvention, according to a first embodiment.

FIG. 3 depicts the coarse approximation of a low frequency signal bymeans of several, equiphase high frequency signals.

FIG. 4 depicts the fine approximation of the difference of the amplituderesponse of the amplified low frequency signal and the coarseapproximation according to FIG. 3.

FIG. 5 is a block diagram of a modulation amplifier, which exhibits theinvention, according to a second embodiment.

FIG. 6 is a block diagram of a radio transmitter with a modulationamplifier according to the invention.

FIG. 7 depicts the output circuit of a radio transmitter, which exhibitsthe invention, according to a first modification.

FIG. 8 depicts the output circuit of a radio transmitter, which exhibitsthe invention, according to a second modification.

The reference numerals, used in the drawings, and their meanings aresummarized in the list of reference numerals. In principle identicalparts are provided with identical reference numerals in the Figures.

METHODS FOR IMPLEMENTING THE INVENTION

A signal of the kind Acos(2 πf₀ t+Φ(t)) can be shown as vectors, whichexhibit the length A and rotate with the angular frequency w₀ =2 πf₀ andthe phase Φ(t).

FIG. 1 shows the vector addition of two signals V1 and V2 with identicalamplitude and frequency, but different phase. V1 corresponds to thesignal s1; V2, the signal s2. ##EQU1##

Thus, the sum of both signals represents a vector V_(R), which rotateswith the angular frequency w₀ and the phase Φ and whose amplitudedepends on the phases of both addends V1 or V2. In the maximum case(Φ1-Φ2=0) the amplitude B becomes twice as large as the originalamplitudes; in the minimum (Φ1-Φ2=180°) B becomes zero.

Thus, the amplitude of the resulting signal can be specifically modifiedby controlling the phases Φ1 and Φ2. This procedure is used for the typeof modulation called "ampliphase".

In so doing, two oscillations with identical frequency f0, but differentphase are added. If the oscillations are imagined as rotating vectors,then it becomes immediately clear that by controlling the phase shift ofthe component vectors the amplitude of the resulting composite vectorcan also be modified. Naturally it can be done in such a manner that theamplitude of the resulting vector changes in accordance with a usefulsignal. The result is an oscillation of the frequency f0, whoseamplitude changes in accordance with the useful signal, in other wordsan amplitude-modulated oscillation.

Assuming that all partial signals are supposed to exhibit the sameamplitude (similar stages), i.e. the ends of the vectors lie on acircle, then it becomes immediately clear that a modulation in the abovecontext can also be performed with a plurality of signals.

It is now known from the EP-B1-0 083 727 that the amplitude response ofa low frequency signal to be amplified and modulated can be approximatedby an addition of multiple, equifrequency and phase-modulated highfrequency sources. The result of such an approximation is a highfrequency oscillation with step-shaped envelop, as shown in FIG. 3.

The idea of the invention is now to approximate the difference of theamplified amplitude response, shown as a dashed line in FIG. 3, and thestep-shaped envelop of the high frequency composite oscillation, shownin FIG. 4, by means of multiple, out-of-phase high frequency sources.

FIG. 2 is a block diagram of a modulation amplifier (1), which can beused for the modulation explained above. Let us call this kind ofmodulation a high frequency pulse stage modulation (HF-PSM), after thepulse stage modulation (PSM), which is known from several older patentsand where direct current sources are added.

Such a HF-PSM modulation amplifier (1) comprises a signal converter (2),a high frequency oscillator (9), a summator (5) and a plurality ofindependently driven high frequency stages (3_(1-m) or 4_(1-n)). A lowpass filter (6) can be attached to the output of the summator (5). Thelow pass filter (6) in turn can be connected to a load (7), shown hereas the antenna.

The high frequency stages (3_(1-m)) are driven by the control signals(s_(1-m)) of the signal converter and emits to its outputs the highfrequency and preferably power-amplified output signals (x_(1-m)). Atthe high frequency stages (4_(1-n)) the corresponding signals are calledt_(1-n) or y_(1-n). These output signals (x_(1-m) or y_(1-n)) are addedin the summator.

A low frequency signal (NF) can be attached to the low frequency input(8) of the signal converter (2). At this stage that number of highfrequency stages (3_(1-m)) can be selected in the signal converter (2)that the oscillation at the output of the summator exhibits astep-shaped envelop, which represents a coarse approximation of theamplified low frequency signal response. This number changes naturallyin accordance with the response to the amplitude of the input signal(NF). The control signals (s_(1-m)) exhibit the frequency f0 and phaseΦ0 of the high frequency oscillator. The output signals (x_(1-m))exhibit the same frequency f0 and phase Φ0 and the same amplitude.

In addition to these equiphase driven stages (3_(1-m)), the highfrequency stages (4_(1-n)) are now driven with different phase, butequifrequency control signals (t_(1-n)). The number of driven stages(4_(1-n)) and the phases Φ_(1-n) are specified in such a manner in thesignal converter that the sum of the output signals (y_(1-n))corresponds to the difference of the amplified low frequency amplituderesponse and the step-shaped envelop of the output signals (x_(1-m)).Thus, the output signals (y_(1-n)) exhibit the same amplitude andfrequency f0, but different phases Φ_(1-n).

Consequently a signal that comprises the coarse approximation by meansof the equiphased coarse stages (3_(1-m)) and the fine approximation bymeans of the different phased fine stages (4_(1-n)) is produced at theoutput of the summator. With the combination of equiphase driven coarsestages and different phase driven fine stages one can obtain arepresentation of the relevant low frequency signal, which is virtuallywithout noise.

As explained above, the number of selected fine stages (4_(1-n)) shouldbe greater than or equal to 2.

In a first embodiment the number of coarse stages (3_(1-m)) is greaterthan that of the fine stages (4_(1-n)); and n is in particular 2.

Another embodiment is characterized in that the number of equiphasedriven high frequency stages (3_(1-m)) is smaller than that of thestages (4_(1-n)) driven with different phases Φ_(1-n). In such amodulation amplifier the stages (4_(1-n)) driven with different phasesassume an increasing larger proportion of the total transmitter power.In the extreme case, shown in FIG. 5, m=1. That is, only a single stageis selected now with control signals, which exhibit the same phase Φ0 asthe high frequency oscillator (9).

The result of such an arrangement is a modulation amplifier, whoseoutput amplitude response is produced by a finite sum of the highfrequency oscillations that exhibit different phases. By determining thephases and the number of selected stages the output amplitude can bearbitrarily modified in accordance with the low frequency input signal.The output amplitude can be adjusted from zero virtually continuously upto a maximum value. The maximum value is obtained by adding the outputsignals of all stages and the equiphase selection. The response of theoutput amplitude becomes thus very smooth and exhibits hardly any steps.Thus, interferences can be largely avoided. Such a modulation amplifieris thus superior to the state of the art with respect to the distortionfactor.

Both embodiments have in common that they can be installed into a radiotransmitter comprising a low frequency signal source (14), a low passfilter (6) for smoothing the transmitting signal and a load (7), usuallyin the form of an antenna. For this purpose the modulation amplifier (1)according to the invention is connected between the low frequency signalsource (14) and the low pass (6). This configuration is shown in FIG. 6.

The summator (5) of the modulation amplifier (1) of the invention isdesigned preferably as a transformer (10). This transformer (10)exhibits per high frequency stage one primary winding (12_(1-m),13_(1-n)) and a secondary winding (11) (see FIG. 8). A secondmodification exhibits n+m transformers with primary windings (12_(1-m),13_(1-n)) and secondary windings (11_(1-m),n) connected in series (seeFIG. 7).

The control signals (s_(1-m) or t_(1-n)) can also comprise physicallines. For example, three lines are conceivable: one, to which the phaseΦ0 of the oscillator is assigned, and two, which are assigned to thepositive or phase shift of the corresponding stage. The signal converter(2) decides then which line is loaded with which signals. The result isthat the stages (3_(1-m) or 4_(1-n)) can also be exchanged arbitrarilywith respect to the physical lines of the control signals (s_(1-m) ort_(1-n)).

Of course, the groups of stages can be divided arbitrarily. Since allstages are constructed in principle the same and are only drivendifferently, when a stage becomes defective, any other stage can assumeits function. Furthermore, it is conceivable that different stages ofone or the other group can belong to different times. A uniform loadingof the stages is also obtained by driving different stages at differenttimes for uniform (also constant) output amplitude values.

Thus, the assignment of stages can be specified arbitrarily in thesignal converter (2). Owing to this condition, owing to the totallysimilar construction of the stages, and owing to the arbitrary selectionby means of the signal converter (2) a modulation amplifier is obtainedthat is characterized by a construction that is extraordinarily flexibleand sturdy.

We claim:
 1. Modulation amplifier comprisinga) a signal converter, whichconverts a low frequency signal, applied to a low frequency input, intoa first group of m(m≧1) control signals (s_(1-m)); b) a high frequencyoscillator, which emits a high frequency signal of frequency f₀ andphase Φ₀ ; c) m high frequency stages (3_(1-m)), which can be drivenindependently and which are driven by the control signals (s_(1-m)) andemit in the turned on state high frequency signals of frequency f₀,phase Φ₀ and identical amplitude to their outputs (x_(1-m)); d) asummator, which adds the outputs (x_(1-m)) of the high frequency stages(3_(1-m)), and sends the composite signal on to a low pass filter, towhose output a load can be attached, whereby e) the signal converterdrives so many high frequency stages (3_(1-m)) that the output signal ofthe summator represents a power-amplified, high frequency oscillation,whose amplitude changes by steps in accordance with the low frequencysignal; and f) means for approximating the difference of the amplifiedamplitude of the low frequency signal and step-shaped output signal ofthe summator; wherein g) the means for approximating the difference ofthe amplified amplitude of the low frequency signal and step-shapedoutput signal of the summator include a second group of n, n≧2,independently driven high frequency stages (4_(1-n)), which h) aredriven by a second group of n high frequency control signals (t_(1-n)),which originate from the signal converter and exhibit the same frequencyf₀, but different phase Φ_(1-n), and i) said high frequency stages(4_(1-n)) send to their outputs (y_(1-n)) high frequency signals offrequency f₀, identical amplitude, but different phase Φ_(1-n), wherebyj) the signal converter determines in such a manner the number ofselected high frequency stages (4_(1-n)) and their phases (Φ_(1-n)) thatthe sum of the output signals (y_(1-n)) of the second group of highfrequency stages (4_(1-n)) represents a high frequency oscillation,whose amplitude corresponds to the difference of the amplified lowfrequency signal and the step-shaped representation of the low frequencysignal by means of the high frequency stages (3_(1-m)).
 2. Modulationamplifier, as claimed in claim 1, wherein the number of high frequencystages (3_(1-m)) is greater or equal to the number of high frequencystages (4_(1-n)), thus m≧n.
 3. Modulation amplifier, as claimed in claim2, wherein the number of high frequency stages (3_(1-m)) is greater thanthe number of high frequency stages (4_(1-n)), thus m≧n, and inparticular n=2.
 4. Modulation amplifier, as claimed in claim 1, whereinthe number of high frequency stages (4_(1-n)) is greater than or equalto the number of high frequency stages (3_(1-n)), thus n≧m. 5.Modulation amplifier, as claimed in claim 4, wherein the number of highfrequency stages (4_(1-n)) is greater than the number of high frequencystages (3_(1-m)), thus n≧m, and in particular m=1.
 6. Modulationamplifier, as claimed in any one of the preceding claims, whereina) theoutput of the high frequency oscillator is connected to the signalconverter; and b) the control signals (s_(1-m)) are high frequency andexhibit the same phase Φ₀ and frequency f₀, like the high frequencyoscillator.
 7. Modulation amplifier, as claimed in any one of thepreceding claims, wherein the summator comprises at least onetransformer.
 8. Modulation amplifier, as claimed in claim 7, wherein thetransformer comprises several primary windings (12_(1-m) or 13_(1-n)),which are assigned to one of the high frequency stages (3_(1-m) or4_(1-n)), and a single secondary winding, which is connected to the lowpass and the load.
 9. Modulation amplifier, as claimed in claim 8,wherein the summator comprises n+m transformers with primary windings(12_(1-m) or 13_(1-n)), which are assigned to one of the high frequencystages (3_(1-m) or 4_(1-n)), and with secondary windings (11_(1-m),n),which are connected in series to the low pass and the load.
 10. A radiotransmitter comprising:a low frequency source; a modulation amplifier; aload; a low pass filter disposed between said modulation amplifier andsaid load; said modulation amplifier including:a) a signal converter,which converts a low frequency signal, applied to a low frequency input,into a first group of m(m≧1) control signals (s_(1-m)); b) a highfrequency oscillator, which emits a high frequency signal of frequencyf₀ and phase Φ₀ ; c) m high frequency stages (3_(1-m)), which can bedriven independently and which are driven by the control signals(s_(1-m)) and emit in the turned on state high frequency signals offrequency f₀, phase Φ₀ and identical amplitude to their outputs(x_(1-m)); d) a summator, which adds the outputs (x_(1-m)) of the highfrequency stages (3_(1-m)), and sends the composite signal on to a lowpass filter, to whose output a load can be attached, whereby e) thesignal converter drives so many high frequency stages (3_(1-m)) that theoutput signal of the summator represents a power-amplified, highfrequency oscillation, whose amplitude changes by steps in accordancewith the low frequency signal; and f) means for approximating thedifference of the amplified amplitude of the low frequency signal andstep-shaped output signal of the summator; wherein g) the means forapproximating the difference of the amplified amplitude of the lowfrequency signal and step-shaped output signal of the summator include asecond group of n, n≧2, independently driven high frequency stages(4_(1-n)), which h) are driven by a second group of n high frequencycontrol signals (t_(1-n)), which originate from the signal converter andexhibit the same frequency f₀, but different phase Φ_(1-n), and i) saidhigh frequency stages (4_(1-n)) send to their outputs (y_(1-n)) highfrequency signals of frequency f₀, identical amplitude, but differentphase Φ_(1-n), whereby j) the signal converter determines in such amanner the number of selected high frequency stages (4_(1-n)) and theirphases (Φ_(1-n)) that the sum of the output signals (y_(1-n)) of thesecond group of high frequency stages (4_(1-n)) represents a highfrequency oscillation, whose amplitude corresponds to the difference ofthe amplified low frequency signal and the step-shaped representation ofthe low frequency signal by means of the high frequency stages(3_(1-m)), wherein the number of high frequency stages (3_(1-m)) isgreater than the number of high frequency stages (4_(1-n)), thus m≧n,and in particular n=2.
 11. A radio transmitter comprising:a lowfrequency source; a modulation amplifier; a load; a low pass filterdisposed between said modulation amplifier and said load; saidmodulation amplifier including:a) a signal converter, which converts alow frequency signal, applied to a low frequency input, into a firstgroup of m(m≧1) control signals (s_(1-m)); b) a high frequencyoscillator, which emits a high frequency signal of frequency f₀ andphase Φ₀ ; c) m high frequency stages (3_(1-m)), which can be drivenindependently and which are driven by the control signals (s_(1-m)) andemit in the turned on state high frequency signals of frequency f₀,phase Φ₀ and identical amplitude to their outputs (x_(1-m)); d) asummator, which adds the outputs (x_(1-m)) of the high frequency stages(3_(1-m)), and sends the composite signal on to a low pass filter, towhose output a load can be attached, whereby e) the signal converterdrives so many high frequency stages (3_(1-m)) that the output signal ofthe summator represents a power-amplified, high frequency oscillation,whose amplitude changes by steps in accordance with the low frequencysignal; and f) means for approximating the difference of the amplifiedamplitude of the low frequency signal and step-shaped output signal ofthe summator; wherein g) the means for approximating the difference ofthe amplified amplitude of the low frequency signal and step-shapedoutput signal of the summator include a second group of n, n≧2,independently driven high frequency stages (4_(1-n)), which h) aredriven by a second group of n high frequency control signals (t_(1-n)),which originate from the signal converter and exhibit the same frequencyf₀, but different phase Φ_(1-n), and i) said high frequency stages(4_(1-n)) send to their outputs (y_(1-n)) high frequency signals offrequency f₀, identical amplitude, but different phase Φ_(1-n), wherebyj) the signal converter determines in such a manner the number ofselected high frequency stages (4_(1-n)) and their phases (Φ_(1-n)) thatthe sum of the output signals (y_(1-n)) of the second group of highfrequency stages (4_(1-n)) represents a high frequency oscillation,whose amplitude corresponds to the difference of the amplified lowfrequency signal and the step-shaped representation of the low frequencysignal by means of the high frequency stages (3_(1-m)), wherein thenumber of high frequency stages (4_(1-n)) is greater than the number ofhigh frequency stages (3_(1-m)), thus n≧m, and in particular m=1.